The use of lasers to melt and recrystallize polysilicon thin films in various silicon semiconducting devices has received increased attention in the semiconductor industry over the past few years. Polysilicon, as deposited using low pressure chemical vapor deposition (LPCVD) or other techniques, has serious drawbacks which limit its use for thin film resistors or active devices such as transistors. The main problem with the deposited film is that it generally consists of many small, randomly aligned crystallites separated by small grain boundries which can be practically considered as defects in the thin film. The grain boundries act as trapping centers, preventing polysilicon films from being used in active areas of devices, such as the base of a bipolar transistor, due to the extremely low minority carrier life time in the polysilicon. The grain boundries are also the cause of a conductive mechanism for majority carriers which is extremely temperature-sensitive, resulting in high negative temperature coefficients when these films are used in thin film resistors, substantially detracting from their value.
One approach to eliminating these problems has been to melt the polysilicon and allow it to recrystallize in larger grains, thus reducing the density of the above-described boundries and improving the electrical characteristics of the film. One method of melting and recrystallizing which has received substantial attention throughout the industry is the use of laser irradiation, particularly continuous wave lasers. Unfortunately, it has been discovered that this technology suffers from its own drawbacks.
One problem encountered in using a scanned continuous wave (CW) laser for melting and recrystallization of polysilicon films is that this technique generally induces severe electrical anisotropy in the film. The conventional technique for scanning the laser beam across the semiconductor wafer is believed responsible for this effect. The laser beam is usually scanned in one direction across the wafer. Since the width of the laser beam is very small, the beam is scanned many times, each time stepping the beam a small distance in a direction perpendicular to the scan, in order to cover the entire wafer. This results in many parallel, closely spaced lines of recrystallized polysilicon grains on the wafer. When a CW laser is used, the polysilicon grains recrytallize in long, thin crystallites in parallel array along the scan lines. It is this structure that is believed responsible for the severe electrical anisotropy found in such films. Electricity flows much more easily along the parallel lines of crystallites generated by the scan line than across them, because the density of the grain boundries across the scan lines is much higher than that along the scan line. The electrical anisotropy induced is a reflection of the spatial anisotropy of the grains generated by parallel laser scan lines.
Accordingly, it is one object of this invention to provide a process for the melting and recrystallization of polysilicon films that does not induce electrical anisotropy into the film.
A second object of this invention is to provide a process of melting and recrystallization of polysilicon that increases the grain size of the recrystallized polysilicon over that obtainable by conventional processes, thereby reducing the sheet resistance of the treated film.
Other objects of the invention will be apparent from its description set forth below.